Key role of eg* band broadening in nickel-based oxyhydroxides on coupled oxygen evolution mechanism

A coupled oxygen evolution mechanism (COM) during oxygen evolution reaction (OER) has been reported in nickel oxyhydroxides (NiOOH)-based materials by realizing eg* band (3d electron states with eg symmetry) broadening and light irradiation. However, the link between the eg* band broadening extent and COM-based OER activities remains unclear. Here, Ni1-xFexOOH (x = 0, 0.05, 0,2) are prepared to investigate the underlying mechanism governing COM-based activities. It is revealed that in low potential region, realizing stronger eg* band broadening could facilitate the *OH deprotonation. Meanwhile, in high potential region where the photon utilization is the rate-determining step, a stronger eg* band broadening would widen the non-overlapping region between dz2 and a1g* orbitals, thereby enhancing photon utilization efficiency. Consequently, a stronger eg* band broadening could effectuate more efficient OER activities. Moreover, we demonstrate the universality of this concept by extending it to reconstruction-derived X-NiOOH (X = NiS2, NiSe2, Ni4P5) with varying extent of eg* band broadening. Such an understanding of the COM would provide valuable guidance for the future development of highly efficient OER electrocatalysts.

Electrochemical water splitting, encompassing the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), constitutes a pivotal technology for addressing the intermittency issues related to renewable energy sources such as solar, wind and tidal power [1][2][3][4] .Among the two reactions, OER is regarded as the bottleneck due to its slow kinetics associated with multiple electron transfer steps and represents a key determinant of the overall energy conversion efficiency into chemical fuels.Thus, the development of high-performance OER electrocatalysts is of paramount importance to advance the field of sustainable energy conversion [5][6][7][8] .Currently, there are two widely recognized mechanisms for the OER process, i.e., the adsorbate evolution mechanism (AEM), a metal redox reaction, and the lattice oxygen oxidation mechanism (LOM), which involves oxygen redox chemistry [9][10][11][12] .The AEM pathway typically involves the ratedetermining step (RDS) of * OOH formation, whereas in the LOM pathway, the deprotonation step serves as the RDS [13][14][15] .The existence of the RDS poses a fundamental challenge to electron transfer efficiency during OER, impeding catalytic performance enhancement.As a result, substantial research efforts have been dedicated to optimizing OER reaction pathways in pursuit of highly efficient electrocatalysts [16][17][18][19] .

Identify COM pathway contributed OER activity promotion
It was revealed that 5% and 20% Fe could be successfully doped into the Ni(OH) 2 lattice 20 .More content of Fe dopants i.e., 40%, 60%, 100% would lead to the aggregation of Fe cations to form Fe 2 O 3 (Supplementary Fig. 1).Since Fe 2 O 3 is a well-studied photocatalyst, its presence might influence the electrochemical results based on the COM.Hence, to focus on investigating the impact of Fe dopants-induced e g * band broadening on the COM, the Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) are chosen as model samples.Fe-removed 1 M KOH electrolyte is employed to minimize the influence of Fe impurity on electrochemical measurements, (the experimental process for Fe removal is described in the Methods section) 21 .The pH value of the purified 1 M KOH is measured to be 13.65 and the error bars represent mean ± standard error (Supplementary Fig. 2). Figure 1a shows the linear sweep voltammetry (LSV) curves of Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) under both light and dark conditions.Negative scan is conducted for the LSV measurement to avoid the influence of Ni 2+/3+ redox current on the OER activity.Hence the negative current peak due to the redox from Ni 3+ to Ni 2+ are observed for Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) (Fig. 1a and Supplementary Fig. 3).Further detailed methodology for measuring the electrochemical activities is provided in the Methods section.Notably, Ni 0.8 Fe 0.2 OOH exhibits the highest OER activity under dark condition, followed by Ni 0.95 Fe 0.05 OOH with the second-highest activity and NiOOH with the lowest activity.For NiOOH under light irradiation, negligible current density variation is observed, which agrees well with our previous research 14 .Interestingly, a significant increase in current density can be observed for Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH under light condition.The overpotential drops for Ni 0.8 Fe 0.2 OOH and Ni 0.95 Fe 0.05 OOH at 10 mA cm −2 between dark and light conditions are 22 mV and 11 mV, respectively (Fig. 1b).To further investigate the contribution of COM to the enhancement of OER performance, the intrinsic activities of Ni 1-x Fe x OOH (x = 0.05, 0,2) are provided with the current density normalized to electrochemical surface area (ECSA, Supplementary Fig. 4) and loading mass.As shown in Fig. 1b, the ECSAnormalized current density difference between light and dark conditions is 0.157 mA cm , prompting geometric transition from NiO 6 octahedra to NiO 4 square planar structures 14 .In high potential region, the limited photon utilization would hinder further contribution of COM to the OER activity, thus serving as the new RDS (Fig. 2a).Therefore, it is of great importance to firstly investigate the two possible RDS in low potential region and high potential region, respectively.The deprotonation of * OH in the COM route closely parallels that in the AEM route, with the primary distinction being the absence of electron transfer during the deprotonation step in the COM pathway 14 .As such, the * OH deprotonation ability in the AEM route could be used to estimate that in the COM route.Therefore, pulse-voltammetry measurements are conducted to investigate the * OH deprotonation ability of Ni 1-x Fe x OOH (x = 0.05, 0,2) within the low potential range (Supplementary Fig. 5, detailed protocol is described in the Methods section).As shown in Fig. 2b, the total stored charge Q ECSA varies linearly with the applied potential for both Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH.During OER process, the oxidative charge is accumulated on the surface of catalysts via the deprotonation step 22 .Hence, the slopes of these fitted lines are calculated to elucidate the rate of oxidative charge to the potential, thereby reflecting the * OH deprotonation ability 20 .The slope value for Ni 0.8 Fe 0.2 OOH is 24.257, which is significantly higher than the slope value of 7.450 for Ni 0.95 Fe 0.05 OOH.This discrepancy suggests that the * OH deprotonation ability of Ni 0.8 Fe 0.2 OOH in the COM route is much stronger than that of Ni 0.95 Fe 0.05 OOH.
Subsequently, we delve into the analysis of photon utilization efficiency.Within the high potential range, the photon utilization to trigger the conversion from NiO 6 octahedron to NiO 4 square planar would gradually become the RDS.In this work, a simulated solar source matching AM 1.5 G was used as the light source with a light irradiation intensity of 100 mW cm −2 .As the potential increases, the COM would become limited due to insufficient photon, leading to the co-existence of COM and AEM route during OER process.Hence, further increasing the applied potential, the promotion of OER activity would be mainly contributed by AEM rather than COM.This could be understood from Tafel slope analysis for Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH in the high potential region, revealing identical slope values under both light and dark conditions (Fig. 2c).Based on the COM route, two photons would participate into the evolution of one oxygen molecule, concomitant with the transfer of four electrons to the external circuit.Therefore, we could estimate the number of participated photons based on the enhanced OER current between light and dark conditions in the high potential region.The photon utilization efficiency hence can be derived by normalizing the number of participated photons to the whole number of photons under 100 mW cm −2 light irradiation (Detailed calculation information is provided in the Methods section and Supplementary Fig. 6).Here, the photon utilization efficiency for Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH is compared at the potential of 1.57 V vs. RHE.The calculated photon utilization efficiency is 0.584% for Ni 0.8 Fe 0.2 OOH, which is nearly 3.3 times than that of Ni 0.95 Fe 0.05 OOH, showing an efficiency of 0.175% (Fig. 2d).
Different e g * band broadening extent in Fe-doped NiOOH system The extent of e g * band broadening in Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) is discussed utilizing a suite of density functional theory (DFT) calculations, Raman spectroscopy, and extended X-ray absorption fine structure (FT-EXAFS) results.Firstly, the DFT calculation is conducted using Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) as computational models (Supplementary Fig. 7) to systematically investigate the effect of Fe dopants on tuning NiO 6 distortion and e g * band broadening.Upon introducing Fe into NiOOH (extract from Ni 0.8 Fe 0.2 OOH model, Supplementary Fig. 7), noticeable extensions of Ni-O bonds along the z-axis are observed, with values increasing from 1.871 Å to 2.034 Å and from 1.894 Å to 1.997 Å, respectively, as compared to NiOOH (Fig. 3a).Then, the e g * band partial density of states (PDOS) of Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) is calculated.As shown in the Fig. 3b, the e g * band PDOS peak becomes broader with lower intensity from NiOOH, Ni 0.95 Fe 0.05 OOH, to Ni 0.8 Fe 0.2 OOH.This trend indicates a stronger e g * band broadening associated with higher concentrations of Fe dopants.Raman spectroscopy of Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) shows two typical peaks at 472 cm −1 and 555 cm −1 (Fig. 3c).Notably, the intensity (I) ratio between these two peaks can be used to identify the crystal structure of NiOOH 23 .The measured I 472 /I 550 value here is 2.81 for NiOOH, 2.07 for Ni 0.95 Fe 0.05 OOH, and 1.40 for Ni 0.8 Fe 0.2 OOH, respectively, indicating a significantly increased level of disorder in the crystal structure of NiOOH with higher Fe dopants concentration.Furthermore, FT-EXAFS analysis demonstrates a reduction in the intensity of the Ni-O bond peak from Ni(OH) 2 , Ni 0.95 Fe 0.05 (OH) 2 , to Ni 0.8 Fe 0.2 (OH) 2 (Fig. 3d).The lower Ni-O bond peak intensity is corresponded to the increased extent of NiO 6 octahedron distortion.Neither the peak intensity around 6 Å nor the Ni-Ni bond peak intensity show great difference, indicating that there is no significant particle size difference among these samples 20 .Hence, the structural difference among these Ni 1-x Fe x (OH) 2 (x = 0, 0.05, 0.2) should be ascribed to the doping effect instead of strain effect.Moreover, the energy difference between d x  -10).This provides strong evidence for the greater e g * band broadening with higher concentration of Fe dopants, which agrees well with our prior reported X-ray absorption spectroscopy (XAS) analysis (Fig. 3e) 20 .Hence, both experimental and theoretical results demonstrate that the introduction of Fe dopants into NiOOH induces varying degrees of NiO 6 octahedron distortion, resulting in distinct levels of e g * band broadening around the Fermi level, with the order being Ni 0.8 Fe 0.2 OOH > Ni 0.95 Fe 0.05 OOH > NiOOH.

Effects of e g * band broadening on COM
As demonstrated, the main structural distinction among Ni 1-x Fe x OOH (x = 0.05, 0.2) lies in the degree of NiO 6 octahedron distortion, leading to varying degrees of e g * band broadening.Moreover, both Ni K-edge and L-edge XAS measurements show that there is negligible structural difference for the samples after OER under dark or light conditions, confirming the reversibility of the electrode surface during the COMbased OER process (Supplementary Fig. 11).In this section, the effect of e g * band broadening on the * OH deprotonation in Ni 1-x Fe x OOH (x = 0.05, 0.2) is investigated.Here, the slope obtained from pulsevoltammetry measurement serves as a representative metric for * OH deprotonation ability, while the white line intensity extracted from Ni K-edge spectra is employed to indicate the degree of e g * band broadening 20 .As shown in Fig. 4a, the Ni 0.8 Fe 0.2 OOH with stronger e g * band broadening shows enhanced * OH deprotonation ability compared to the Ni 0.95 Fe 0.05 OOH.Meanwhile, the calculated reaction free energies of the * OH deprotonation process for Ni 0.8 Fe 0.2 OOH following the COM route is also lower than that of the Ni 0.95 Fe 0.05 OOH, which is consistent with the pulse-voltammetry results (Supplementary Fig. 12).The enhanced proton transfer ability ultimately results in a substantial improvement in the OER activity under light irradiation within the low potential region, where the * OH deprotonation serves as the RDS (Fig. 4b).
With the applied potential increased, the COM-based OER activity would be limited by the photon utilization efficiency.Our prior research demonstrated that the non-overlapping region between d z 2 and a 1g * orbitals played a key role in the photon absorption process, which was greatly related to the e g * band broadening 14 .To unveil the underlying science of the extent of e g * band broadening and enhanced OER activities in the high potential region, the relationship between the e g * band broadening and the photon utilization efficiency is investigated.In theory, if the non-overlapping region between d z 2 and a 1g * orbitals is broader, it would make the electron transfer from (M-O) to d z 2 easier under light irradiation.This, in turn, would result in a higher photon utilization efficiency.As shown in Fig. 4c, the d z 2 orbital for NiOOH is completely overlapped with the a 1g * orbital.Hence, electrons would be unfeasible to transfer from (M-O) to d z * band broadening induced by Fe dopants (Fig. 4d).Subsequently, the relationship between the broadness of nonoverlapping region and the corresponding photon utilization efficiency is analyzed.As shown in Fig. 4e, with the increase of broadness of the non-overlapping region, the photon utilization efficiency is significantly promoted from 0.175% to 0.584%.To further confirm such effect, LSV measurements of Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH under light with wavelengths of 575 nm, 475 nm, and 365 nm are performed.The Ni 0.8 Fe 0.2 OOH with stronger e g * band broadening exhibits higher current density promotion at 1.57 V vs. RHE between dark and light conditions at each wavelength compared to Ni 0.95 Fe 0.05 OOH (Supplementary Fig. 13).From a calculated perspective, the reaction free energy of the light-dominated step for Ni 0.8 Fe 0.2 OOH is 0.89 eV, much lower than that of the Ni 0.95 Fe 0.05 OOH (0.94 eV) (Supplementary Fig. 12).As such, these results prove that increasing the non-overlapping region via tuning e g * band broadening would facilitate the electron transfer from (M-O) to d z 2 under light irradiation, leading to higher photon utilization efficiency (Fig. 4f).could be induced by both cation dopants and strain effect 20 .Hence, to validate the proposed hypothesis, it is necessary to verify this in systems influenced by strain effects.Specifically, the reconstruction derived X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ) samples are prepared through the electro-oxidation of NiS 2 /NiSe 2 /Ni 5 P 4 at a current density of 10 mA cm −2 for 10 h, which would exhibit different extent of e g * band broadening due to the strain effect, with the following order: NiS 2 -NiOOH>NiSe 2 -NiOOH >Ni 5 P 4 -NiOOH >NiOOH 20 .Here, the OER activities of X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) are investigated under both light and dark conditions.As shown in Fig. 5a, discernible discrepancies could be observed in the degree of activity enhancement among X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) between light and dark conditions.NiS 2 -NiOOH displays the most substantial improvement in OER activity under light irradiation, with an overpotential drop of 23 mV at 10 mA cm −2 .In comparison, NiSe 2 -NiOOH exhibits a lower degree of OER activity promotion, with a 15 mV reduction in overpotential, but a higher promotion compared to Ni 5 P 4 -NiOOH, which shows an 11 mV overpotential reduction.As such, a positive correlation between the extent of e g * band broadening and the enhanced OER activities is revealed (Fig. 5b).At the same time, both the * OH deprotonation ability and photon utilization efficiency of X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) follow the same order: NiS 2 -NiOOH>NiSe 2 -NiOOH >Ni 5 P 4 -NiOOH (Fig. 5c, d, Supplementary Fig. 14).These results demonstrate the universal concept that increased e g * band broadening could simultaneously facilitate * OH deprotonation and promote photon utilization, resulting in highly efficient COM-based OER activity.

Discussion
In this work, Ni 1-x Fe x OOH (x = 0, 0.05, 0.2) samples are prepared to investigate the role of e g * band broadening on enhancing the COMbased OER activity.The results demonstrate that a stronger e g * band broadening would not only facilitate * OH deprotonation, but also promote photon utilization efficiency by generating a broader non-overlapping region between d z 2 and a 1g * .As such, both of the two RDS in the COM route could be effectively alleviated by realizing stronger e g * band broadening, resulting in higher OER activity.Furthermore, the universality of this concept is demonstrated by revealing the same effect on promoting the COM-based OER activity in X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) with different e g * band broadening extent.Previously, it was also unveiled that the stronger e g * band broadening would trigger the emergence of more electronic states around the Fermi level.This effect notably facilitated the electron transfer from electrocatalysts to external circuit via * OH deprotonation, resulting in higher OER activities (AEM-pathway) 20 .As for the LOM route, the deprotonation step is usually the RDS.The conceptual framework encompassing NiO 6 distortion, e g * band broadening, and * OH deprotonation also holds the potential for application in the context of the LOM process, thereby enabling the realization of enhanced catalytic performance.Therefore, it is believed that increasing the degree of e g * band broadening could be a crucial factor on promoting the participation of COM under light irradiation and even provide valuable guidelines for future design of highly efficient OER catalysts following various reaction pathways.
Synthesis of the Ni 1-x Fe x (OH) 2 (x = 0, 0.05, 0,2) The Ni 1-x Fe x (OH) 2 (x = 0, 0.05, 0,2) were grown on the carbon cloth by a facile hydrothermal treatment 20 .Before hydrothermal treatment, the carbon cloth was pre-treated under 500 °C for 1 h in the air condition and then dealt with Ultra-Violet Ozone device for 30 min to make it fully hydrophilic.Next, 2 mmol total amount of Ni(NO) The X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) was obtained from the reconstruction of NiS 2 , NiSe 2 , and Ni 5 P 4 via a 10 h chronopotentiometry treatment 20 .First, one piece of hydrothermal-derived Ni(OH) 2 loaded carbon cloth was placed at the downstream position in crucible, while 400 mg sulfur/selenium/sodium phosphate monobasic power was placed at the upstream position.Then, the crucible was kept at 400 °C for 2 h under N 2 atmosphere and then cooled down to temperature.After that, the synthesized NiS 2 /NiSe 2 /Ni 5 P 4 was washed using DI water and ethanol for at least three times, and dried at 70 °C for 1 h.Next, the chronopotentiometry treatment was conducted under 10 mA cm −2 for 10 h to fully transfer the NiS 2 /NiSe 2 /Ni 5 P 4 to X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ).

Removal of Fe impurity
The 1 M KOH solution was purified to remove Fe impurity before electrochemical measurement.To be specific, 0.5 g Ni(NO) 3 •6H 2 O powder was added into 30 mL 1 M KOH solution, forming Ni(OH) 2 precipitation.Next, the suspension was centrifuged at 7826 × g for 10 minutes.Then, the obtained Ni(OH) 2 powder was added into 50 mL KOH solution and mechanically agitated for 10 min.After standing still for 24 h, the suspension was centrifuged at 7826 × g for 10 minutes, and the KOH supernatant was kept in a clean electrochemical cell for use.The pH value of the purified 1 M KOH is detected to be 13.65 and the error bars represent mean ± standard error (Supplementary Fig.

Material characterizations
The Raman spectra were recorded by a Raman Spectrophotometer with an excitation wavelength of 514.4 nm.Nickel K-edge X-ray absorption fine structure (XAFS) spectra were recorded at the XAFCA beamline at the Singapore Synchrotron Light Source (SSLS) under transmission mode where the storage ring is running at 0.7 GeV with current nearly 200 mA 24 .The energy calibrations were finished by using standard Nickel foil.The k 2 -weighted Fourier transforms were conducted using the Hanning window function for the EXAFS results, with the k-range of 2.5-10.5 Å −1 .

Electrochemical measurements
All the electrochemical measurements were performed using an electrochemical workstation (VPM3, BiO-logic Inc) in a three-electrode setup in 1 M KOH.The working electrode was the Ni 1-x Fe x OOH (x = 0, 0.05, 0,2) and X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ).The Hg/HgO was chosen as the reference electrode.The Pt was used as the counter electrode.The linear scan voltammetry (LSV) was measured at scan rate 0.1 mV s −1 .In the dark situation, all electrochemical measurements are conducted in a dark box.Under light condition, a solar simulator (NBeT Solar-500, 300 W) was used as the light source to match AM 1.5 G.The intensity of the light irradiated on the electrocatalyst was 100 mW cm −2 as calibrated by a light power meter (Newport 843-R).The experimental set-up is provided in Supplementary Fig. 15.Before evaluating the OER activity, all samples were subjected to chronopotentiometry measurements at a current density of 10 mA cm −2 under dark for 24 h to ensure the complete oxidation of Ni 2+ to Ni 3+ .

Pulse-voltammetry measurement
One small piece of electrocatalysts on carbon cloth (0.2 cm 2 ) after electrochemical oxidation was used for the pulse-voltammetry measurement.The potential was set firstly at a low potential (E l = 1.40 V) for 80 s to stabilize the electrode surface.Then the potential was turned to a higher potential (E h = 1.42 V) for 12 s and back to E l for 12 s as one cycle.This cycle was repeated while increasing E h from 1.42 V to 1.48 V versus RHE in 20 mV/step with constant E l = 1.40 V.The transferred charge normalized to ECSA during each cycle was evaluated by integrating the current pulse/ ECSA over time.

Electrochemically surface area
The electrochemically surface area (ECSA) data was evaluated from recording the electrochemical double-layer capacitance of the catalyst via cyclic voltammograms (CVs).Here, the potential range was set at 0.02-0.12V (versus Hg/HgO) to avoid the Faradaic process.The CVs were conducted in the quiescent electrolyte with the potential swept across the set potential range with at 6 scan rates 5, 10, 20, 30, 40, 50 mV s −1 .The charging current was plotted versus scan rate and a straight line could be derived with the slope value equalled to the double-layer capacitance (C dl ).The ECSA was obtained by dividing the C dl to the specific capacitance C s = 0.04 mF cm −2 25 .

Photon utilization calculation
In COM route, two photons participated into one oxygen molecule evolution, which corresponded to four electrons.Hence, the utilized photon number was calculated by the equation: in which ΔJ was the current density difference between light and dark conditions at 1.57 V vs. RHE, t was reaction time, S was surface area, e was elementary charge.The number of photons per second and surface unit for certain wavelength was calculated by where I [W m −2 nm −1 ] was the irradiance for certain wavelength, which could be known from Supplementary Fig. 5a; E P was the photon energy, h was Planck constant, c was speed of light, λ was wavelength.
The N P -λ spectra could therefore be obtained, as shown in Supplementary

Computational method
All calculations were conducted via the DFT with the generalized Perdew-Burke-Ernzerhof (PBE) 26 , and the projector augmented-wave (PAW) 27 pseudopotential planewave method as implemented in the Vienna ab initio Simulation Package (VASP) code 28 .For the PAW pseudopotential, 3d 8 4s 2 , 3d 7 4s 1 , 2s 2 2p 4 and 1s 1 were treated as valence electrons for Ni, Fe, O and H atoms, respectively.The bulk β-NiOOH structure was optimized within the local-spin-polarized density approximations (LSDA + U, U eff = 5.3 eV for Ni and 5 eV for Fe, respectively) .The plane wave cutoff energy of 500 eV was set for all calculations and the energy and force convergence were set to 10 −5 eV and 0.02 eV Å −1 , respectively.Grimme's DFT-D3(BJ) dispersion correction 29 were used in all calculations.A 12 × 12 × 10 Monkhorst-Pack (MP) K-point grid was used for β-NiOOH unit cells geometry optimization calculations.The layered NiOOH structure was optimized using the same basic settings of the bulk β-NiOOH structure optimization except the k-point sampling (12 × 6 × 1).For COM route, the thermodynamic correction of Gibbs free energy was implemented by VASPKIT 30 and the relative energy was calculated by: ΔGð2Þ = G *OO ð ÞÀGðO*OÞðLight dominated processÞ ð 5Þ

KFig. 1 | 2 (
Fig. 1 | Electrochemical characterization of Ni 1-x Fe x OOH (x = 0, 0.05, 0.2) for the OER under dark and light condition.a LSV polarization curves of Ni 1-x Fe x OOH (x = 0, 0.05, 0.2) based on a backward scan conducted at a scan rate of 0.1 mV s −1 under dark (dash line) and light (solid line) condition (without iR-correction).b Comparison of the overpotential drop (Δη) at a current density (J) of 10 mA cm −2(purple color), the current density improvement normalized to ECSA (ΔJ ECSA ) at an overpotential of 300 mV (cyan color), and the current density improvement normalized to loading mass (ΔJ Mass ) at an overpotential of 300 mV (orange color), respectively for Ni 1-x Fe x OOH (x = 0.05, 0.2).

8 8 1 Fig. 2 |
Fig. 2 | Analysis of * OH deprotonation and photon utilization efficiency for Ni 1-x Fe x OOH (x = 0.05, 0.2). a Schematic illustration of the rate-determining step in low potential region and high potential region under COM route.b Charge versus potential for Ni 1-x Fe x OOH (x = 0.05, 0.2) from pulse-voltammetry (without iR-correction).c Tafel slope for Ni 1-x Fe x OOH (x = 0.05, 0.2) in high potential region.d Photon utilization efficiency for Ni 1-x Fe x OOH (x = 0.05, 0.2) at 1.57 V vs. RHE.

2 .
After doping Fe into NiOOH, a distinctive non-overlapping region emerges between d z 2 and a 1g * for both Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH, indicating the possible electron transfer under light irradiation.More importantly, the broadness of the non-overlapping region increases from 0.65 eV for Ni 0.95 Fe 0.05 OOH to 0.84 eV for Ni 0.8 Fe 0.2 OOH.The broadening of the non-overlapping region could be ascribed to the stronger extent of e g

Fig. 5 |
Fig. 5 | Effects of increasing e g * band broadening extent on COM-based OER activity of X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ).a LSV polarization curves of X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ) based on a backward scan conducted at a scan rate of 0.1 mV s −1 .(withoutiR-correction) b The correlation between the COM contributed overpotential drop at 10 mA cm −2 and the Ni K-edge white line intensity of X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ).c The correlation between the Ni K-edge white line intensity with the * OH deprotonation ability of NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ).d The correlation between the Ni K-edge white line intensity and the photon utilization efficiency of X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ).It should be noted that the white line intensity of X-Ni(OH) 2 (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) is extracted from Fig. 3a in our previous work 20 to evaluate the e g * band broadening for X-NiOOH to avoid the possible effects on the XAS results caused by the selfdischarge of Ni 3+31 .

N p =
I=E p = I × λ=hc = I × λ × 10 À9 =ð1:988 × 10 À25 Þ = I × λ × 5:03 × 10 15 ½Photons m À2 s À1 nm À1 Fig. 5b.Integration of the N P -λ spectra yields a theoretical maximum photon flux N of 4.936 × 10 22 photons m −2 s −1.Hence, the photon utilization efficiency was calculated asN 0 =N Total = ð0:5ΔJ × t × S=eÞ=ðN × t × SÞ = 0:5ΔJ=ðN × eÞð3ÞArticle https://doi.org/10.1038/s41467-023-43302-2NatureCommunications | (2023) 14:7488 −2for Ni 0.8 Fe 0.2 OOH and 0.053 mA cm −2 for Ni 0.95 Fe 0.05 OOH at an overpotential of 300 mV.Meanwhile, the loading mass-normalized current density improvement is 15.846A g −1 for Ni 0.8 Fe 0.2 OOH and 4.159 A g −1 for Ni 0.95 Fe 0.05 OOH, respectively.Based on these results, it can be concluded that the intrinsic OER activity increment of Ni 0.8 Fe 0.2 OOH under the COM route is greater than that of Ni 0.95 Fe 0.05 OOH.In our previous investigation, we have established that the COM predominantly governs the electrochemical process in low potential region, where there is an ample supply of photons to initiate electron transfer from (M-O) to d z 3•6H 2 O and Fe(NO 3 ) 3 •9H 2 O, 10 mmol urea were added into 35 mL deionized (DI) water and stirred for 15 min to form uniform solution.One piece of carbon cloth with size of 2 cm × 3 cm was immersed into the solution.Then, they were transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept in oven at 120 °C for 10 h.The obtained Ni 1-x Fe x (OH) 2 (x = 0, 0.05, 0,2) was then washed by DI water and ethanol for at least three times, and further dried under 70 °C in air for 4 h.Synthesis of the X-NiOOH (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) The optimized unit cell of layered NiOOH was expanded to 6 × 3 supercells (a = 17.556Å and b = 17.739Å) containing 72 Ni, 72 H and 144 O atoms with a vacuum thickness of 18 Å for Fe embedding systems.Based on our experimental results, two different Fe doping concentrations were considered in this study, they are Ni 0.95 Fe 0.05 OOH and Ni 0.8 Fe 0.2 OOH corresponding to computational models of Fe 4 Ni 68 and Fe 14 Ni 58 , respectively (Supplementary Fig.7, the CONTCAR of the optimized Ni 1-x Fe x OOH (x = 0, 0.05, 0.2) models is provided in Supplementary Data 1).It should be noted that we constrained lattice constants and relaxed all atoms in Fe embedding systems simulations.For supercell case, only γ K-point was considered.